[3.12] gh-128588: gh-128550: gh-128552: fix refcycles in eager task creation (#128553) and remove eager tasks optimization that missed and introduced incorrect cancellations (python#129063)

[graingert]

(cherry picked from commit 61b9811)

• Issue: TaskGroup's task's exception's participate in a reference cycle, via TaskGroup.create_task -> task -> exception and loop.create_task -> task -> exception #128552

• Issue: TaskGroup fails to cancel tasks pending creation, if that task starts a failing task #128550

• Issue: Cancellation leaks out of asyncio.TaskGroup on 3.12 when using eager tasks #128588